When a parameter is marked as optional with ? it indicates that callers can omit the argument when calling the function. If another parameter is required after the optional parameter, the ? would be effectively invalidated since users must pass the argument in order to provide the later required argument.
function test(a?: number, b: number) {}Fix: Allow the argument to be undefined.
Explicitly union the first argument with undefined and omit the question mark:
function test(a: number | undefined, b: number) {}Fix: Re-order parameters
Reorder the parameters so that required parameters appear before the optional ones:
function test(b: number, a?: number) {}Related: TS1015